Conventionally, as a power supply including a rectifying and smoothing circuit based on the current doubler rectification method of the above-mentioned kind, a power supply 81 shown in FIG. 17 is known. This power supply 81 includes a switching transformer 2, and a current doubler rectifying and smoothing circuit 82. In this case, the current doubler rectifying and smoothing circuit 82 is comprised of a smoothing choke coil 14 connected between one end 2b1 of a secondary winding 2b of the transformer 2 and an output terminal 16b on a low-potential side, a smoothing choke coil 15 connected between the other end 2b2 of the secondary winding 2b and an output terminal 16b on a high-potential side and having the same inductance value as that of the choke coil 14, a diode 11 as a rectifying element, connected between the one end 2b1 of the secondary winding 2b and an output terminal 16a, and a diode 12 as a rectifying element, connected between the other end 2b2 of the secondary winding 2b and the output terminal 16a. The current doubler rectifying and smoothing circuit 82 outputs a DC voltage V0 generated by rectifying and smoothing a bipolar voltage induced between the opposite ends of the secondary winding 2b to a load 4.
In this power supply 81, push-pull FET circuits, not shown, connected to one end 2a1 of a primary winding 2a of the transformer 2 and the other end 2a2 thereof, respectively, are driven at 180 degrees out of phase with respect to each other, whereby as shown in FIG. 18, a bipolar voltage VS having a voltage value.+-.VS is induced between the opposite ends of the secondary winding 2b of the transformer 2. In this case, in a period T1 during which one of the FET circuits is controlled to an ON state at a duty ratio D of 25%, a high voltage is induced on the side of the one end 2b1 of the secondary winding 2b during the ON time period TON of the FET, and this induced voltage causes a current I31 shown in FIG. 17 to flow through a current path of the one end 2b1 of the secondary winding 2b, the diode 11, the load 4, the choke coil 15, and the other end 2b2 of the secondary winding 2b. In this state, as shown in FIG. 18, a voltage VL15 having a voltage value (VS-V0=(1-D)/D-V0/f, where f represents a frequency of the bipolar voltage VS) and directed as shown in FIG. 17 is generated between opposite ends of the choke coil 15, whereby energy is accumulated in the choke coil 15.
Further, during an OFF time period TOFF of the period T1, the energy accumulated in the choke coil 15 causes a current I32 to flow in a direction shown in the same figure through a current path of one end of the choke coil 15, the diode 12, the load 4, and the other end of the choke coil 15. Consequently, the voltage VL15 between the opposite ends of the choke coil 15 is caused to have a voltage (-V0), and at the same time, as shown in FIG. 18, a current IL15 varying within a range of a current variation width ((VS-V0).cndot.TON/Lo=(1-D).cndot.V0/f, where Lo represents an inductance value of the choke coils 14 and 15) flows through the choke coil 15.
Further, in the period T2 (the same time period as the period T1) during which the other FET is controlled to an ON state at a duty ratio D of 25%, a high voltage is induced on the side of the other end 2b2 of the secondary winding 2b during the ON time period TON of the FET, and this induced voltage causes a current I33 shown in FIG. 17 to flow through a current path of the other end 2b2 of the secondary winding 2b, the diode 12, the load 4, the choke coil 14, and the one end 2b1 of the secondary winding 2b. In this state, as shown in FIG. 18, between the opposite ends of the choke coil 14 is generated a voltage VL14 having a voltage value (VS-V0) and directed as shown in FIG. 17, whereby energy is accumulated in the choke coil 14.
Further, during an OFF time period TOFF of the period T2, the energy accumulated in the choke coil 14 causes a current I34 to flow in a direction shown in FIG. 17 through a current path of one end of the choke coil 14, the diode 11, the load 4, and the other end of the choke coil 14. Consequently, the voltage VL14 between the opposite ends of the choke coil 14 become equal to a voltage value (-V0), and as shown in FIG. 18, a current IL14 varying within a range of a current variation width ((VS-V0).cndot.TON/Lo=(1-D).cndot.V0/f) flows through the choke coil 14. In the above process of operation, each of average current values of the currents IL15 and IL14 becomes equal to one half of an output current I0, since a sum total of the current values of the currents becomes equal to the output current I0, shown in FIGS. 17 and 18, and at the same time the current values thereof are equal to each other. It should be noted that as shown in FIGS. 17 and 18, a ripple current IC flowing through the capacitor 13 varies within a range of a current variation width ((1-2D).cndot.V0/f=(1-TON/(T-TON)).cndot.(VS-V0).cndot.TON/Lo, where D represents a duty ratio, and f represents the reciprocal of the period T).
As described above, smoothing operations are carried out by the choke coils 14 and 15 during a time period of each of the periods T1 and T2, so that as shown in FIG. 18, an output current Io from which a ripple component is substantially eliminated is output to the load 4.